Energy-absorbing and force-limiting friction coupling

ABSTRACT

A device having kinetic energy-absorbing properties is connectable between selected points within a structure, for example a building within a seismic area. When one connectable part moves against another during an earthquake, friction is ensured by tension implanted internally at the time of manufacture, when one part of the device had been yieldingly forced into place in relation to another part. When in use the device provides a damping force should a force applied b etween its parts be greater than or equal to a predetermined force at which sliding friction begins.

This invention relates to coupling devices for absorbing unwanted kinetic energy by passively converting kinetic energy into heat and for limiting forces by slipping at a predetermined load. More particularly the invention relates to passive energy dissipation devices for use in protecting structures such as buildings, for reducing damage during earthquakes.

DEFINITIONS

“Residual tension.” In this invention, the residual tension is a force developed as a sequel of a stretching type of distortion that is implanted within the device at the time of manufacture and maintained indefinitely by virtue of principally elastic and related properties of the materials involved.

“Residual compression.” In this invention, the residual compression is a force developed as a sequel of a compressional type of distortion that is implanted within the device at the time of manufacture and maintained indefinitely by virtue of principally elastic and related properties of the materials involved.

“Hookean material.” In this text, steel is described as an elasto-plastic material. Hooke's law is an approximation which states that the amount by which a material body is deformed (the strain) is linearly related to the force causing the deformation (the stress) or, f=−kx. Steel exhibits a linear Hookean region (wherein displacement and force are linearly and reversibly related) up to an upper limit, beyond which it enters a ductile (plastic) region that is used in this invention, while even more deformation is followed by rupture. See FIG. 3.

BACKGROUND

Artificial structures such as buildings, bridges or other structures that are erected in sites where earthquakes comprise a risk may include means to prevent damage that would otherwise be a consequence of ground movement during a “significant” earthquake by which we mean one that could cause structural damage to buildings. Any earthquake is of limited although unpredictable duration and amplitude related to the relatively unpredictable amount of pent-up energy to be released. Given that unpredictability, inclusion of means to prevent excessive movement (of a substantially elastic type) may on occasion be insufficient to prevent damage, but at least the risk is reduced. In the absence of dedicated devices adapted for the dissipation of energy, some portions of a structure exhibit inelastic behaviour and may damp movement to some extent but usually this is at the cost of permanent damage or failure to those parts of the structure.

Another approach is to introduce ductility into a structure by using a dedicated device. Typical forces such as those caused by internal loading, design wind loading, and thermal expansion or contraction would not be sufficient to activate the means to resist yet allow movement. It is usual for such means to resist a force of less than a given threshold without inelastic deformation, and beyond that threshold to deform in a way which absorbs energy, dampens the motion caused by the force, and limits the forces transmitted to other parts of the structure.

A means to resist but allow limited movement of the structure exposed to seismically induced shaking should provide damping of possible resonance in the structure (in any mode). This is desirable because resonant movement may exaggerate the initial magnitude of vibration and could lead to collapse of the structure.

A means to resist but allow controlled movement should desirably be designed so that that the forces transmitted to any individual element are limited to less than that which would cause failure of the element. The means should also be designed so that the forces transmitted to any individual element are limited and so accelerations in the structure are also limited.

Passive energy absorbing systems have previously been described for damping motion, relative to a substrate, in buildings. A 36-page review by Aiken I D et al in Earthquake Spectra vol 9, no. 3, (August 1993, from the Earthquake Engineering Research Institute, California) described seven different passive energy dissipation systems; three being Coulomb friction systems, one, the “Fluor-Daniel (Inc) Energy Dissipating Restraint being a form of sliding friction restraint with a stop; also a system using ADAS (added damping and stifffness) steel elements, one using viscoelastic shear dampeners, and one using nickel-titanium alloy shape memory devices.

Sliding friction dampers, such as that described in U.S. Pat. No. 5,560,162 to Kemeny, use a seismic brake comprising a shaft journalled through a split gripping block which frictionally engages the shaft. The gripping block is provided with a liner, and the friction between the liner and the shaft can be adjusted by adjusting the clamping force created by the gripping blocks. The clamping force is created by bolts drawing the two halves of the gripping block together thus producing a predominantly uni-axial clamping action on the shaft. This solution may work well, but may be unnecessarily complicated and/or expensive, and calibration of the device may be difficult and unreliable. Further, if a particular friction is specified by a design engineer the field-adjustability of this device may allow it to be changed when in use so that the properties of the damper are outside the specifications.

The reference to any prior art in the specification is not, and should not be taken as, an acknowledgement, or any form of suggestion, that the prior art forms part of the common general knowledge in any jurisdiction.

OBJECT

It is an object of the invention to provide a coupling device for absorbing unwanted kinetic energy two points in a structure by passively converting kinetic energy into heat, or which will at least provide a useful choice. Other objects of the present invention may become apparent from the following description, which is given by way of example only.

STATEMENT OF INVENTION

In a first broad aspect, the invention provides an energy-absorbing and force-limiting friction coupling device for reducing damage arising from movements induced within artificial structures including buildings by events such as earthquakes, wherein the device has a first member extending therefrom and terminating with a first attachment means at an end for fastening to a first structural element, and a second member extending therefrom and terminating with a first attachment means at an end for fastening to a second structural element; a frictional region is provided within the device; the frictional region is occupied by a friction-capable portion of the first member and by a friction-capable portion of the second member, the second member being capable of undergoing frictional, sliding movement over a distance at the frictional region relative to the first member; and wherein friction at the frictional region is ensured by including a clamping means at least partly comprised of a material having elasto-plastic properties about the frictional region for clamping the members together in frictional contact.

Preferably, the clamping means is capable of being pre-loaded with a residual tensile force by imposition of a force sufficient to enter the ductile range of the clamping means and cause yielding to occur within the clamping means so that, when in use, a force of at least a predictable magnitude applied between the first and the second attachment means will cause frictional, sliding movement over a distance and conversion of kinetic energy into heat within the device at the frictional region.

In a first related aspect the invention provides an energy-absorbing and force-limiting friction coupling device—wherein the clamping means includes a maximised implanted tensile force capable of keeping the first and second members in frictional contact; the tensile force having been implanted within the clamping means at the time of manufacture of the device and having been maximised at the time of manufacture by deforming the material having elasto-plastic properties to at least the point of yield.

In a related aspect, the invention provides an energy-absorbing and force-limiting friction coupling device wherein the frictional region includes a friction-enhancing material attached to one member and capable of sliding along a portion of the other member.

In one option, the friction-enhancing material attached to one member is in the form of a hollow cylinder and exists in a state of radial tension while forcibly stretched around and and in frictional contact with a parallel-sided shaft forming the friction-capable portion of the other member.

In another option, the friction-enhancing material attached to one member is forcibly enclosed within and exists in a state of radial compression and in frictional contact with the other member; the friction-capable portion of the other member comprising an elongated parallel-sided cavity enclosing the friction-enhancing material.

In a furher option, the friction-enhancing material is identical with the clamping means (such as, for example, phosphor bronze which is a strong copper alloy).

In another related aspect the invention provides an energy-absorbing and force-limiting friction coupling device wherein the frictional region includes apposed frictional surfaces each comprised of a metal.

Preferably, a first frictional surface is comprised of a metal selected from the range including copper and alloys of copper.

Preferably also, a second frictional surface is comprised of a steel.

Optionally a lubricant, selected from a range including graphite and plated lead, is included at the time of manufacture.

In a second broad aspect the invention provides a method of making an energy-absorbing and force-limiting friction coupling device as previously described in this section, wherein the residual tensile force is implanted within at least the elastic portion of the clamping means of the device during a process of assembly; during which process a selected shaft comprising the friction-capable part of the first member is forced past the friction-capable part of the second member through an under-sized space or aperture; the shaft dimension being selected so that a force applied to the clamping means during the assembly process is sufficient to exceed the yield strength of the clamping means such that at least a portion of the clamping means of the device is caused to enter the ductile range, so that a maximised tensile force present within the elasto-plastic clamping means shall be consistently applied to the frictional area over time.

Preferably the amount of force applied to the clamping means during manufacture is recorded as a descriptive property applicable to that device during use.

More particularly, the method includes at least one step that pre-selects parts having specified force characteristics and comprises the steps of:

(a) applying the following approximate formula when selecting parts to be assembled:

Sliding Force=2·μ·L·π·t·F _(y)

-   -   Where: L=the length of the friction-enhancing material (assumed         to be in the shape of an annulus);     -   μ=the coefficient of friction of the friction-enhancing material         against the friction-capable portion of the other member;     -   t=the thickness of the annulus;     -   F_(y)=the yield stress of the annulus.     -   r_(f)=the radius of the friction surface of the annulus

(b) placing the parts so selected within a press, and

(c) forcing the parts together.

In a related aspect the method also provides an equivalent formula for use in rotational relative movement.

Where . . . (with the same variables)

Rotational Torque=2·πr _(f) μ·L·t·F _(y)

In a third broad aspect the invention provides an overload release coupling wherein the coupling is capable of transmitting a torque without loss as long as the torque remains below a level corresponding to the commencement of sliding friction, whereupon the overload release coupling will commence to dissipate at least some of the torque in the form of heat.

Preferred Embodiment

Further aspects of the invention, which should be considered in all its novel aspects, will become apparent from the following description given by way of example of possible embodiments of the invention. The description of the invention to be provided herein is given purely by way of example, and is not to be taken in any way as limiting the scope or extent of the invention.

Throughout this specification, unless the text requires otherwise, use of the word “comprise” and variations such as “comprising” or “comprises” will be understood to imply the inclusion of a stated integer or step or groups of integers or steps but not the exclusion of a stated integer or step or groups of integers or steps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2: Diagrams showing how a residual tension maybe implanted within a kinetic energy-Absorbing and force-limiting coupling according to the invention, at the time of assembly.

FIG. 3: Graph of stress v strain for a typical elasto-plastic material, to identify the properties discussed in the accompanying specification with reference to the invention.

FIG. 4: Perspective sectioned view of a kinetic energy absorbing and force limiting connecting device based on FIGS. 1 and 2.

FIG. 5: Perspective view, in section, of an “inside out” version of FIG. 4.

FIG. 6: Perspective view, in section, of a furter kinetic energy absorbing and force limiting connecting device with bilayered frictional member and seals.

FIG. 7: Cross-sectional view of a kinetic energy absorbing and force limiting connecting device intended for service as a type of emergency release coupling for a rotating shaft. FIG. 8: Perspective view, in section, of a furter kinetic energy absorbing and force limiting connecting device. The frictional portion can be regarded as a collar over a bolt. There is a stop preventing excessive movement in either direction.

FIG. 9: Perspective view, in section, of a further kinetic energy absorbing and force limiting connecting device; an insert suitable for embedment in suitably reinforced concrete.

FIG. 10: Perspective view, in section, of a further kinetic energy absorbing and force limiting connecting device; an insert suitable for attachment to reinforcing bars.

FIG. 11: Force (X axis) v displacement (Y axis) graph showing conversion of kinetic energy into heat by a device according to the invention (FIG. 4).

FIG. 12 (as 12 a and 12 b): Diagram showing how a tendency of a vertical structure to sway and be distorted, as in an earthquake, is damped by means of the invention placed between beams and columns.

FIG. 13: Diagram of an implementation for steel beams of the concept of FIG. 12 b.

FIG. 14: Diagram showing how a tendency of a vertical structure to rock on its foundations, as in an earthquake, is damped by means of the invention (the FIG. 9 example, for instance).

FIG. 15 (as 15 a and 15 b): Diagram showing how swaying movement of a vertical structure having intentionally provided sliding tracks upon rigid braces may be damped by means of the invention.

BEST MODES FOR PERFORMING THE INVENTION

Finished forms of the invention are sometimes referred to as “KEA” devices.

EXAMPLE 1 Predominantly Sliding Motion

FIGS. 1, 2, and 4 show a conceptual version of this device comprising two parts; a rod 100 that may slide inside a tube 300 which tube for the purpose of illustration includes a portion 200, a “frictional mass” that is adapted to exhibit friction against the rod. The device uses substantial internal friction to impede relative movement of the two parts along or about the axis of the rod. Both static friction and sliding friction are applicable. It should be noted that if the dimensions of the rod alter along its length then the amount of sliding friction obtained will not be constant. A diameter tolerance of about 1 part in 500 is quite adequate. The invention is intended to be installed so that it connects one element of a structure to another using conventional connecting means projecting from each end, where if the force applied to the device is great enough, relative movement within the device occurs; and heat is generated as a result of the built-in friction.

When one item is intended to slide against another item, in a mode wherein an amount of friction between the items is desired, there may be a requirement to force the two objects against each other using a deliberately applied and controlled force. The force may arise from an existing force such as gravity, alternatively use of a screw, lever or other machine that amplifies an existing force, or in some cases the force arises from (for example) magnetic attraction, or tension or compression applied through a spring.

In this invention, the force on the frictional interface or surface that is relied upon is caused by a “residual tension” (or residual compression), which is a desired sequel of a supra-yield force that had been applied to one or more of the parts of the device generally referred to herein as a “clamping means” at the time of manufacture. It is maintained indefinitely by virtue of elasto-plastic properties (in particular) of the parts involved when under tension or compression. Steel is a preferred material for the parts involved.

During construction, the two parts which are designed to be overly tight mis-fits are assembled (pressed) together preferably with sufficient force to exceed the elastic region of the Hooke's law curve and cause eventual yielding (often with visible, permanent deformation) of one or more parts—though preferably not to such an extent that the strength of that part is compromised in any way for example by rupture. FIG. 1 shows a shaft 100 and a friction-generating ring 200 before assembly. The diameter of the hole is made deliberately smaller than the diameter of the shaft. Assembly comprises forcing, as shown by arrows 201, the deliberately too-small ring 200 over the tapered section 101 of the shaft and on to the body of the shaft (or, of course, the converse, forcing the tapered shaft inside the ring). So much axial force is used during forcing that the frictional member 200 is dilated as a whole. Either the frictional member itself or clamping means surrounding the frictional member (sleeve or cylinder 300) is plastically deformed by the force required for passage of the over-sized shaft. The zone of contact pressure between the ring 200 and the shaft 100, usefully enhanced by the residual tension, is the region where static and sliding friction is present. In FIG. 2, the ring, which had to be stretched in order to fit over the shaft, now includes tensile forces (arrows 202, the residual tension,) that tend to restore the diameter of the ring towards its initial size. During manufacture, the tapered end 101 of the shaft is forced into the opening 302 and through a hole 203 in frictional member 200 and then it is left in place.

The example of FIGS. 1, 2 and 4 is not the only possible way to make the invention. For example, FIG. 5 shows a ring 200A that had been deformed by entry into a relatively under-sized enclosing tube 100 provided with a taper 301A at the entry, so that residual compression within the ring seeks to expand the ring against the inside of the tube 100 and thereby promote friction at the zone of contact. The ring is held within a circumferential notch 30 1A. Practical forms of the invention (FIGS. 4-9) are designed so that the residual tension force has the effect of enhancing a frictional effect at a surface of a frictional mass 200. As a result of stretching the ring (or an outer support such as a stretched portion 301 of hollow tubular section 300—FIG. 4) beyond its elastic limit the tensile force 202 exerted at the surface that develops friction is as large as could conveniently be applied in a passive device without some form of mechanical amplification of the force.

FIG. 3 shows a conventional stress (Y)/strain (X) or force (Y)/displacement (X) graph 310 for a an elasto-plastic material such as steel. The stress or force (vertical axis) represents the tension 202 in the ring-like component 200 in FIG. 2, while the strain or displacement can be related to the amount of distortion caused during assembly. The steeply rising part of the curve from the graph origin 311 to 312 is the elastic range; the section from 312 to 313 is the ductile range where inelastic, yielding deformation occurs, and beyond 313 rupture of the material may be expected. Note that subsequent relaxation of an already yielded substance will traverse another linear curve, now offset towards the right as shown as line 315-316 and the residual force obtained under a pre-existing geometry will be relatively limited. Hence, effects such as corrosion or unauthorised re-use should be monitored and controlled. The extent of mis-fitting built into devices according to the invention (as in the style of FIG. 4) is intended to result in causing deformation to for example 4% elongation, at about band 314 on the graph. The approximately horizontal slope of the curve in the yielding region (312-313) confers a useful tolerance on the actual extent of mis-fitting. If the relevant step of the assembly failed to exceed the yield strength (i.e. failed to pass point 312), then the force applied across the frictional surface is (a) less, and (b) less predictable or consistent, although the resulting devices do retain some utility. In that case the applicable part of the curve is relatively vertical. Residual tension in that case is highly dependent on the actual force used during assembly. This invention preferably enters the ductile range of the elasto-plastic material used in order to create a reliable amount of residual tension. Should versions of the present invention be made wherein the elastic limit of deformation of at least one part of a device was not exceeded during manufacture, such devices will have reduced utility since the maximum amount of residual tensional or compressional force within the clamping means will not have been obtained, and since the amount of force actually implanted will be more difficult to determine.

The above concept is put into practice in FIG. 4 as follows: Member 100 (an optionally hollow shaft or solid rod having a tapered end 101 at one end and suitable connecting means at the other end (not illustrated here) comprises one sliding part of the device; while the other sliding part that develops friction against the first part in the event of sufficient force being applied to cause motion is made up of tubular (hollow) member 300 having an opening 302 and dilated area 301, inside which frictional member 200 is held tightly against shaft 100 at a friction-generating zone between 200 and 100. The frictional member 200 is restrained from moving up and down by conventional means such as (a) the restricted dimensions of the area that has been deformed, 301, or (as in FIG. 5) use of a holding notch 301A, or (as in FIG. 6) use of a shaped cylinder 303 with a restricted opening 302 which is welded to shaft/tube 300 after inclusion of a frictional member 200—here a composite made up of a frictional layer 202 inside a collar or sleeve 301. (Likewise, connection means for the second part is not shown here).

It is usual to verify after the assembly process that the distortion was consistent and no rupture as such had occurred, because that would weaken the final amount of radial force. The part undergoing inelastic deformation is the friction material and the outer band or sleeve which is preferably steel since that material has good properties; better than those of some frictional materials. Provided that the frictional material itself has adequate properties, it could be used without a second band or sleeve. For example, some types of phosphor bronze have adequate properties. In practice, more than one part will experience tensile forces after the manufacturing process is completed, although only one part is likely to exhibit deformation. These parts include (a) the frictional member 200 and/or (b) the containing outer wall dilated at 301, and/or (c) the shaft 100 inside the frictional member. The frictional member 200 could be deliberately weakened so that it does not significantly contribute to the clamping force. In that case, the outer sleeve 301 receives all the distorting force and subsequently the residual tensile force within it closes the frictional member tightly around the shaft 100, applying all the force at the frictional surface that has to be overcome during passive energy conversion into heat.

The resulting properties of the invention as a device passively converting kinetic energy into heat by means of “Coulomb-like friction” are therefore determined at least approximately by (a) dimensions, and (b) materials, so long as at least the yield strength was exceeded and some visible distortion exists, so that inherent tensional forces remain after assembly. Under small amounts of applied force the invention transmits the force without itself undergoing movement of one part relative to another. If the force is or becomes great enough to overcome static friction, relative movement occurs; meanwhile much energy is converted into heat. In the event of the force starting to change direction, as is likely in the case of earthquake waves, static friction again becomes dominant until the force has changed direction and again exceeds the limitation of static friction. A plot of force against displacement will provide a rectangular hysteresis curve (See FIG. 11) in the form of a closed loop. A desired feature of such dampers is to have a large area enclosed by the plot, partly by appropriate selection of materials and partly through physical design. In FIG. 11, a trial of a device resembling a FIG. 4 version of the invention and using a frictional combination of cartridge brass against steel began with an increasing negative (vector) force applied at the upper right corner. A small amount of displacement only was seen until the trace entered its horizontal phase moving towards the left while the negative force was applied, but with no real change in the amplitude of the force. During that period, sliding friction was present and the force was being converted into heat (Coulomb-like friction) proportional to the area enclosed by the trace. When the force reversed in direction at lower left, static friction again became dominant until the applied force again exceeded the static friction, and the trace moved substantially horizontally to the right. The trace formed a loop because, when the applied force cycled between a negative and a positive direction, static and then sliding friction existed in turn. In an event of overheating from excessive earthquake energy, the device will approach a melting point, will stop exerting frictional effects and will allow the structure under protection to move more freely. An in-service design should therefore ensure that there is sufficient thermal mass in the device(s) to prevent overheating. A trial of a similar device having a phosphor bronze frictional device showed a much reduced shrinking loop effect.

EXAMPLE 2 Predominantly Rotational Motion

We now consider versions of the invention for use in limiting the amount of torque that may be applied along a rotating shaft. In this application the invention serves as an overload release coupling. This Example is manufactured in the same general manner as for the previous Example, again using the principle of forcing one member past another member in an assembly procedure using deliberately mis-fitting parts that results in causing distortion within the invention. As a result, a clamping means is constructed within the device and is pre-loaded with tension (or compression in some versions) that has the effect of clamping the two members tightly together but will nevertheless allowing sliding friction to occur if the torque applied to one member relative to the other member exceeds a predetermined limit. Indeed, the same device (such as is illustrated in FIG. 4 or 5) may be used, made and pre-loaded with residual tension or compression in the same way, modified only by using a selected rotationally compatible coupling. FIG. 7 is a cross-section through a simplified form and practical devices might for example be much shorter. The axis of rotation is the dash-dot line 701. A spline coupling is provided at each end (702 and 703). Other parts are labelled as elsewhere. Other suitable couplings are well-known in the art, such as a keyway, or a gear or a pulley held by a screw impinging on to a flat on a shaft. The manufacturing process preferably verifies that the torque-limiting device, after the forcible assembly step that should involve at least some deformation (but might not be taken into the yield region), does retain reasonably concentric axes of rotation 701 for both the first member (100) and the second member (300, so that the device runs true). A device of this type may serve to absorb unexpected movement in both rotational and translational directions. The transfer function would be as follows: the output tracks the input, revolution for revolution, in either direction unless the maximum transmissible torque is exceeded, whereupon the output slips and a difference between input power and output power is converted into heat. This invention has the advantages that in at least one form it is compact and has pre-determined and reasonably constant properties. In general, compact forms which have a relatively small heat capacity ought not to be used in applications where large amounts of power are likely to be converted into heat, and should be reserved for emergency use.

Applications include vehicles of all types, mowers, agricultural and earth-moving machinery, conveyers, crushers, shredders, propellor drives for boats, food mixers, printing and other machinery, and door and gate operating devices, with a great number of other relevant applications also available.

A method for making rotatable couplings of this type may employ the following simplified formula in the pre-selection of parts having specified torque characteristics:

Rotational Torque=2·πr _(f) μ·L·t·F _(y)

-   -   Where: L=the length of the friction-enhancing material (assumed         to be in the shape of an annulus);     -   μ=the coefficient of friction of the friction-enhancing material         against the friction-capable portion of the other member;     -   t=the thickness of the annulus;     -   F_(y)=the yield stress of the annulus.     -   r_(f)=the radius of the friction surface of the annulus.

After selection, the parts are pressed together, preferably while monitoring the force used (as described elsewhere) so that individual devices may be supplied in a pre-calibrated form.

EXAMPLE 3 Some Applications

FIG. 8 shows a bidirectional version of the “Kea” device that may be passed through a vertical beam (enclosed by washers 802, 802A), and its central shaft 100A—100 may be attached at both ends by screw threads with nuts 801, 801A to, for example, horizontal beams. The nuts 801, 801A may also serve as stops.

FIGS. 9 and 10 illustrate two versions of the “Kea” device suitable for use with reinforced concrete, where for example they may serve to reduce sway of a structure built on top. The FIG. 9 version is made with a belled-out base 900 that will tend to resist being pulled from cured concrete, especially if a reinforcing bar parallel to but beneath the surface is passed through the aperture 901. The FIG. 10 version is more suited to being welded (1002) by its tubular end 1000 on to a stout bar 1001—and this version need not be within a mass of concrete.

FIG. 12 (as 12 a) shows a frame structure 1200 and the type of movement (dashed outline) seen in an earthquake, while FIG. 12 b shows an example installation in elevation detail where “Kea” devices (1201) may be placed between a stiff vertical beam 1202 and a stiff horizontal beam 1203. FIG. 13 is a practical elevation drawing of an installed “Kea” device according to FIG. 12 b. FIG. 14 illustrates how a rigid block structure 1401 might rock upon a rigid base 1402 during an earthquake, although such movement would be impeded by use of “Kea” devices 900 (such as those of FIG. 9). FIG. 15 a shows a structure 1500 using rigid triangular braces 1501 in combination with a sliding brace and slide bearings 1502 that support the horizontal beams 1203 at each storey, and FIG. 15 b shows how a “Kea” device 1201 may be used to dampen sideways movement in the event of an earthquake.

Friction-Generating Materials

The mass or surface against which friction is developed should be dimensionally stable at all times prior to a seismic event and until movement begins, so that the device retains its original calibrated characteristics. The possibility of creep or flow over a period of perhaps many years is likely to exclude flowable frictional materials having a plastics or composite basis, or lead. Copper and various alloys including copper, such as brass, cartridge brass, bronze, silicon bronze, phosphor bronze, and gunmetal are at present being tested in combinations; generally working against steel surfaces. The friction means may be relatively thick, for improved strength during manufacture and greater constancy of properties during use over long periods, or may be relatively thin (down to a few mm or less, perhaps sufficient only for use in one earthquake event in order to reduce cost. Only a part and possibly only a thin layer 201 of the frictional member 200 as drawn in FIG. 6 may be made of a material having primarily frictional properties, and the remainder may be, for example, a steel. Optional provision of a lubricating means is contemplated. A lubricant may comprise a graphite-based lubricant, use of a brass-like friction device, or covering the rod with a layer of electroplated lead, and the benefits may be a more even frictional behaviour or less noise during operation. Corrosion protection may be an extra benefit. There is a possibility that a device containing dissimilar metals in contact will exhibit a raised rate of corrosion. In the event of corrosion attacking the steel shaft or tube that comprises one member, it is likely that the corroded material will be scraped off during operation of the device having the result that the effective diameter of the shaft or tube will be smaller. Because subsequent relaxation of an already yielded substance will (with reference to FIG. 3) traverse another linear curve, now offset towards the right as shown as line 315-316 the residual force that had been set up under a previous geometry will not be recovered, hence it is desirable that corrosion of devices according to the invention is prevented. The inventors propose sealing the opening 302 (see FIG. 4) in order to exclude oxygen with (for example) a two-pot epoxy resin mixture at the time of manufacture and advise re-sealing units after an earthquake has fractured each opening (which serves as a telltale). Alternatively an O-ring seal (such as that shown in section at 601 in FIG. 6) may be installed in each unit for repeated use without attention, or the units may simply be maintained in a substantially dry atmosphere.

Process for Selection and Manufacture

We shall assume that the design of the constructed building has been resolved to an extent wherein devices according to the invention will be used between specified positions and will each have a specified first force requirement to overcome static friction, and then a specified force during dynamic friction, which are ideally equal. A design engineer will be able to issue specifications for devices according to the invention.

Typically, the manufacturer proceeds to fill the order by first employing the following approximate formula in order to select parts having suitable characteristics in relation to the threshold force (or torque) at which the kinetic energy absorbing and force limiting connecting means will begin to slip.

Sliding Force=2·μ·L·π·t·F _(y)

Alternatively, (for a rotating relative movement)

Rotating Torque=2·π·r _(f) ·L·μ·t·F _(y)

Where: L=the length of the friction-enhancing material (assumed to be an annulus);

-   -   μ=the coefficient of friction of the friction-enhancing material         against the friction-capable portion of the other member;     -   t=the thickness of the annulus;     -   F_(y)=the yield stress of the annulus; and         -   r_(f)=the radius of the friction surface of the annulus.

In some embodiments a lubricant, preferably a graphite lubricant, may be provided between the annulus and the shaft. In such embodiments the variable , in the equations above represents the coefficient of friction of the friction means material against the elongate member with the lubricant present. The formula prescribes components having dimensions capable of performing as required. The formula shown here is an approximation. For example it does not attempt to locate the radius of the mean residual tension force, although such enhancements are available in the literature.

Manufacture

The basic parts (shafts and tubes) that the inventors have used in experimentation are compliant with the well-known ISO standard “H9 tolerances” or within 0.64 mm deviation in 50 mm and may be held in stock or obtained as needed. Such tolerance regimes are not critical although more consistently shaped articles do of course provide a more constant relationship between force and heat. It is likely that an initial step in adoption of these devices is the specification of customised sets of KEA devices by an engineer according to shape geometry and the force required to initiate sliding friction, based on that engineer's understanding of the behaviour of structures during applied forces such as during an earthquake. Next, the approximate formula given above is invoked in order to calculate particular dimensions of the parts, given known materials characteristics, and the selected or sized parts are brought to an assembly area. Finally, devices are assembled (pressed) from parts as previously described in this section, and preferably each device is subjected to quality control comprising (a) unique identification, (b) measurement of properties during assembly, and (c) recording those properties (the assembly force required) on or along with each device. Visual inspection is of course also used in case of unexpected rupture. An anti-corrosion treatment such as sealing, as previously described, may also be applied. Finished, characterised devices are then consigned to the construction site. It will be noted that the invention does not provide for field adjustment, short of disassembly, re-sizing, and reassembly. This is likely to comprise an advantage, since the devices cannot be altered beyond the control of the design engineer.

Data describing the assembly force required for each individual device may be obtained in proportional form from a hydraulic line feeding a hydraulic piston performing the pressing operation or it may be directly obtained in absolute terms from a force transducer built into the press. Such data may be printed on to a durable label along with dates, variants, batch numbers and serial numbers in order to provide a high degree of quality control and traceability.

Variations

Although it is convenient to provide the shaft 100 and the other parts of the device in circular section form as shafts or tube forms, and these are particularly easy to machine on a lathe, it is of course feasible to use other profiles such as square section—which in some instances would offer the benefit of preventing mutual rotation of parts during use.

Since a long service life is desired, in some environments extra steps may be taken to minimise corrosion. Had the shaft 100 diameter been effectively reduced by corrosion over time, one might expect the relaxation of the device back into the elastic region of deformation to occur along another path (see FIG. 3) resulting in reduced force-dependent properties. Selection of suitable materials is but one step. A frangible seal over the aperture 302 of the embodiment of FIG. 4 may serve to exclude oxygen from the interior.

The tapered end 101 of rod 1 requires a machining step. The taper may be provided at the time of manufacture only—as a type of durable hole-widening shaft termination that is capable of being pressed into a too-small aperture to be widened, and is later recovered after pressing for re-use. It may be made of an optimised, hard yet not brittle material, for the purpose.

The invention can be supplied in “turnbuckle” form—in which a mounting thread at one end has a reverse thread as compared to a mounting thread at the other end. After an event, the “KEA” invention may become fixed in a different amount of extension to that present at installation and this if not adjusted may cause perceptible distortion of non-structural elements.

Other uses of the kinetic energy absorbing and force limiting connecting means are also envisaged, for example wharf fenders, vehicle impact barriers, and the like.

INDUSTRIAL APPLICATIONS AND ADVANTAGES

The “KEA” device is cheaper than existing equivalents, in part because of the simple structure, ease of manufacture, and reduced consumption of non-ferrous or expensive materials.

The device can be made with a technology no more complex than that used in the manufacture of bolt-based linkages used in construction. The tapered commencement of the shaft that is used to introduce the wider portion into the device during manufacture may be a re-usable part, made of a hard-wearing material.

Cheapness permits use of more “KEAs”, or the replacement of some or all of them (if accessible) after a major event.

Inclusion of a residual tension or compression has no effect on matters such as handling characteristics.

The “KEA” invention is supplied individually pre-calibrated, and tested, and cannot be adjusted after manufacture, so that a design engineer can be confident that the dynamic behaviour of a structure including the “KEA” invention is and will remain predictable within specifications.

The “KEA” invention does not require major alterations to a design for a structure when earthquake resistance is added. The devices can be included within an existing part.

Where in the foregoing description, reference has been made to specific components or integers of the invention having known equivalents, then such equivalents are herein incorporated as if individually set forth. Although this invention has been described by way of example and with reference to possible embodiments thereof, it is to be understood that modifications or improvements may be made thereto without departing from the spirit or scope of the invention as provided within the appended claims. 

1-15. (canceled)
 16. An energy-absorbing and force-limiting friction coupling device for reducing damage arising from movements induced within artificial structures including buildings by, events such as earthquakes, characterized in that the device includes a first member extending outwards from the device and terminating with a first attachment means at a first end for fastening to a first structural element, and a second member extending outwards from the device and terminating with a second attachment means at a second end for fastening to a second structural element; the first and second members being held in contact at an intermediate frictional region occupied by a slidable portion of the first member and by a slidable portion of the second member by a force forcing the first and second members into factional contact; the force originating within a clamping member exhibiting elastic (Hookean) and ductile (plastic) properties capable of holding the slidable portions in tight contact after having been plastically deformed during manufacture by a process of forcing together the slidable portion of the first member and the slidable portion of the second member despite misfitting dimensions; the clamping member being capable of continuing to apply a substantial portion of the force across the factional region between the friction-capable portion of the first member and the friction-capable portion of the second member for an extended period; so that, when in use, a force having more than a predictable magnitude applied between the first and the second attachment means will cause frictional, sliding movement over a distance and conversion of kinetic energy into heat within the device at the frictional region.
 17. An energy-absorbing and force-limiting friction coupling device as claimed in claim 16, characterized in that the friction-capable-portion of the first member and the friction-capable portion of tine second member are forced into contact with each other by a manufacturing process causing elastic and then plastic distortion of one member so that after the force of the manufacturing process has ceased, the respective members remain strongly forced together by substantially all of the force that had been required to cause elastic distortion so mat, when in use, a force having at least a predictable magnitude applied between the first and the second attachment means will cause frictional, sliding movement over a distance and conversion of kinetic energy into heat within the device at the frictional region.
 18. An energy-absorbing and force-limiting friction coupling device as claimed in claim 17, characterized in that the frictional region includes a friction-enhancing material attached to one member and capable of sliding along a portion of the other member.
 19. An energy-absorbing and force-limiting friction coupling device as claimed in claim 18, characterized in that the friction-enhancing material attached to one member is in the form of a hollow cylinder and exists in a state of radial tension while forcibly stretched around and and in frictional contact with a parallel-sided shaft forming the friction-capable portion of the other member,.
 20. An energy-absorbing and force-limiting friction coupling device as claimed in claim 18, characterized in that the friction-enhancing material attached to one member is forcibly enclosed within and exists in a state of radial compression and in frictional contact with the other member; the friction-capable portion of the other member comprising an elongated parallel-sided cavity enclosing the friction-enhancing material.
 21. An energy-absorbing and force-limiting friction coupling device as claimed in claim 19, characterized in that the friction-enhancing material is the same component as the clamping means.
 22. An energy-absorbing and force-limiting friction coupling device as claimed in claim 20, characterized in that the friction-enhancing material is the same component as the clamping means.
 23. An energy-absorbing and force-limiting friction coupling device as claimed in claim 19, characterized in that the frictional region includes apposed frictional surfaces each comprised of a metal.
 24. An energy-absorbing and force-limiting friction coupling device as claimed in claim 20, characterized in that the frictional region includes apposed frictional surfaces each comprised of a metal.
 25. An energy-absorbing and force-limiting friction coupling device as claimed in claim 23, characterized in that a first frictional surface is comprised of a metal selected from the range including copper and alloys of copper.
 26. An energy-absorbing and force-limiting friction coupling device as claimed in claim 23, characterized in that a second frictional surface is comprised of a steel.
 27. An energy-absorbing and force-limiting friction coupling device as claimed in claim 23, characterized in that a lubricant, selected from a range including graphite and plated lead, is included at the time of manufacture.
 28. A method of making an energy-absorbing and force-Limiting friction coupling device as claimed in claim 16, characterized in that the residual tensile force is implanted within at least the elastic portion of the clamping means of the device during a process of assembly; during which process a selected shaft comprising the friction-capable part of the first member is forced past the friction-capable part of the second member through an under sized space or aperture; the shaft dimension being selected so that a force applied to the clamping means during the assembly process is sufficient to exceed the yield strength of the clamping means such that at least a portion of the clamping means of the device is caused to enter the ductile range, so that a maximised tensile force present within the clamping means shall be consistently applied to the frictional area over time,
 29. A method of making an energy-absorbing and force-Limiting friction coupling device as claimed in claim 27, characterized in that the amount of force applied to the clamping means during manufacture is recorded as a descriptive property applicable to that device during use.
 30. A method of making an energy-absorbing and force-Limiting friction coupling device adapted mainly for linear motion as claimed in claim 28, characterized in that the method includes at least one step that pre-selects parts having specified force characteristics and comprises the steps of: (a) applying the following formula when selecting parts to be assembled: Sliding Force (N)=2·μ·L·π·t·F _(y) where: L=the length in mm of the friction-enhancing material (assumed to be an annulus); P1 μ=the coefficient of friction of the friction-enhancing material against the friction-capable portion of the other member; t=the thickness of the annulus (mm); F_(y)=the yield stress of the annulus (in MPa). r_(f)=the radius of the friction surface of the annulus (mm) (b) placing the parts so selected within a press, and (c) forcing the parts together.
 31. A method of making an energy-absorbing and force-Limiting friction coupling device adapted mainly for linear motion as claimed in claim 29, characterized in that the method includes at least one step that pre-selects parts having specified force characteristics and comprises the steps of: (a) applying the following formula when selecting parts to be assembled: Sliding Force (N)=2·μ·L·π·t·F _(y) where: L=the length in mm of the friction-enhancing material (assumed to be an annulus); μ=the coefficient of friction of the friction-enhancing material against the friction- capable portion of the other member; t=the thickness of the annulus (mm); F_(y)=the yield stress of the annulus (in MPa). r_(f)=the radius of the friction surface of the annulus (mm) (b) placing the parts so selected within a press, and (c) forcing the parts together.
 32. A method of making an energy-absorbing and force-limiting friction coupling device as claimed in claim 30 though adapted mainly for rotational motion, characterized in that the method includes pre-selection of parts having specified force characteristics and comprises the steps of: (a) applying the following formula when selecting parts to be assembled: having specified torque characteristics: Rotational Torque=2·πr _(f) μ·L·t·F _(y) where: L=the length of the friction-enhancing material (assumed to be in the shape of an annulus); μ=the coefficient of friction of the friction-enhancing material against the friction- capable portion of the other member; t=the thickness of the annulus; F_(y)=the yield stress of the annulus. r_(f)=the radius of the friction surface of the annulus. (b) placing the parts so selected within a press, and (c) forcing the parts together.
 33. An energy-absorbing and force-limiting friction coupling device manufactured according to claim 32 and adapted mainly for rotational motion, characterised in that the coupling device is provided with one co-axial coupling means at each end of the device and that the coupling device will faithfully couple axial rotation from one end to the other unless the relative torque applied between one co-axial coupling means and the other exceeds a predetermined limit. 